This particular site http://www.inquisitr.com/ is apparently getting about 7 million page views a month with a eCPM of about $2.21
($15,449 / 7,000,825 page views) * 1,000 = $2.21

The site has numerous ad slots which take a fair amount of real estate on the page so it's not the best match for my original question, but does give a slight indication. It also has affiliate income so this skews the figure a little also.

[It isn't really possible to answer this too well without more detailed information from you, at which point the question could become too specific, but:]

You might start to get a general sense of ranges by using the "browse" filters at AdRatesOnline.
One caveat, however, is that I've never even heard of the majority of sites listed there. Sites which have established a "name" for themselves in their field will obviously have more room to negotiate for more, and will also be more likely to have their own advertising pages set up with their rates. You'd probably be better off looking at your specific competitors, if their ad information is openly available.

Like everyone else said, it's a tough question to answer. From what I've seen, ad networks often advertise a range from .20 cents to 15$ eCPM, sometimes 25$ eCPM.

The best way to figure out would be to keep your stats at hand and shop some ad networks around. Any ad network should be able to give an estimate to you by checking other clients' records against your own. If they don't, they're probably not worth your time.